Mesoscale Precipitation Discussion 0791
NWS Weather Prediction Center College Park MD
202 PM EDT Fri Sep 07 2018

Areas affected...West, Central and North Texas, and Southern
Oklahoma

Concerning...Heavy rainfall...Flash flooding likely

Valid 071801Z - 080000Z

Summary...Areas of rain with embedded heavier rain bands or
thunderstorms are expected to continue through the afternoon along
and north of a stationary front from Texas into southern Oklahoma.
This is likely to lead to areas of flash flooding this afternoon.
Rain rates of 1-2 in/hr will be common in the heavier rain bands,
with localized rates to around 3 in/hr possible in more extreme
circumstances. Some of the flash flooding could be more widespread
or severe where heavy rain bands stall for an extended period of
time.

Discussion...There is increasing confidence in heavy rainfall and
more widespread flooding issues this afternoon from as far west as
the Stockton Plateau and Permian Basin in West Texas, extending
northeast along a front into Central and North Texas, and near the
Red River in southern Oklahoma. Convective activity was embedded
in a larger area of light to moderate rain along and north of a
stationary front. The embedded convective rain bands and a few
thunderstorms did not have a great vertical depth, and were
largely dominated by warm rain processes. This is to be expected
with a deep moisture plume, melting levels around 14-15kft, and
tall, narrow CAPE profiles -- a classic combination for efficient
rain production and collision-coalescence processes. Dual pol data
from area radars also tended to confirm this, with reduced ZDR --
indicative of smaller, spherical rain drops more common through
the collision-coalescence process.

Two areas of more concentrated convection and heavy rainfall are
currently expected. One is from near the Midland-Odessa area east
and northeast to near Abilene and Throckmorton. The convective
bands appeared to be stalling southeastward progress in this area
in a low-level deformation zone. Via radar loops, there also
appeared to be a weak, broad MCV or mesolow in the low levels just
to the west of Midland, which could be locally enhancing
convergence and increasing the low-level deformation along the
front in the 925-850mb layer where the convective bands are more
common. There are multiple Flash Flood Warnings already in effect
in these areas and stalling cell motions and training convection
may cause an expansion of the flash flood threat and exacerbating
flash flood impacts in areas already being affected.

Further to the northeast, there was an area of enhanced low-level
convergence where northerly flow around the west side of Tropical
Depression Gordon was meeting broad southerly flow over East Texas
in the warm sector. This was setting up an additional boundary
from south-central Oklahoma into far Northeast Texas, and this
could focus additional convective development in the next few
hours.

Outside of those two areas, flash flooding will remain possible,
but convective coverage may be a little more isolated. However,
the broader environment with PWATs generally in excess of 2 inches
(above the 90th percentile for September in this region) should be
supportive of heavy rain rates and flash flooding. Deep layer mean
winds and Corfidi vector magnitudes across the entire region are
generally 10 knots or less.

Lamers

ATTN...WFO...EWX...FWD...LUB...MAF...OUN...SHV...SJT...TSA...

ATTN...RFC...ABRFC...LMRFC...WGRFC...

LAT...LON   34839767 34669537 34059492 33289571 32499681
            31949787 31259893 30439977 30050100 30090199
            29860348 30880385 32180279 32870121 33729945